Estou usando a classe phpmailer para enviar e-mails. Atualmente, o Gmail e o Yahoo não marcam os emails como spam, mas o Hotmail sempre o faz. Como posso evitar isso? Meu código está abaixo.

require_once('../PHPMailer/class.phpmailer.php');
//include("class.smtp.php"); // optional, gets called from within class.phpmailer.php if not already loaded

$mail = new PHPMailer();

$mail->IsSMTP();    // set mailer to use SMTP
$mail->Host = "mail.example.com";    // specify main and backup server
$mail->SMTPAuth = true;    // turn on SMTP authentication
$mail->Username = "xxx";    // SMTP username -- CHANGE --
$mail->Password = "xxx";    // SMTP password -- CHANGE --
$mail->Port = "25";    // SMTP Port

$mail->From = "[email protected]";    //From Address -- CHANGE --
$mail->FromName = "xxx";    //From Name -- CHANGE --
$mail->AddAddress($email, $passerusername);    //To Address -- CHANGE --
$mail->AddReplyTo("[email protected]", "xxx"); //Reply-To Address -- CHANGE --

$mail->WordWrap = 50;    // set word wrap to 50 characters
$mail->IsHTML(false);    // set email format to HTML

$mail->Subject = "AuthSMTP Test";
$mail->Body    = "AuthSMTP Test Message!";

if(!$mail->Send())
{
   echo "Message could not be sent. <p>";
   echo "Mailer Error: " . $mail->ErrorInfo;
   exit;
}

echo "Message has been sent";